Duplex stainless steels have a mixed structure of body-centered cubic (bcc) ferrite and face-centered cubic (fcc) austenite. The exact amount of each phase is a function of composition and heat treatment. Most alloys are designed to contain about equal amounts of each phase in the annealed condition. The principal alloying elements are chromium and nickel, but nitrogen, molybdenum, copper, silicon, and tungsten may be added to control structural balance and to impart certain corrosion-resistance characteristics.
The corrosion resistance of duplex stainless steels is like that of austenitic stainless steels with similar alloying content. However, duplex stainless steels possess significantly higher tensile and yield strengths, typically above 60 ksi (425 MPa), and improved resistance to stress-corrosion cracking than their austenitic counterparts. The toughness of duplex stainless steels is between that of austenitic and ferritic stainless steels.
Duplex stainless grades are used in applications that take advantage of their superior corrosion resistance, strength, or both. Typical applications include:
So-called lean duplex steels are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. Superduplex steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels. Duplex stainless steels are weldable, but welding consumables and heat input must be controlled to maintain the ferrite-austenite balance.